It is known that the person only uses a grip force slightly larger than the minimum grip force to lift an object with his/her finger, even though the friction factor is unknown. To let a robot do this gripping motion, the robot needs to be provided with a tactile sensor capable of detecting a friction factor and/or a “slippage margin” indicating how easily a gripped object slips. There are following developments in regard to such a tactile sensor. One is a tactile sensor to be mounted in a fingertip of a robot. This tactile sensor senses a friction factor of a gripped object, by actually letting the object slip. Another development is a tactile sensor or a friction factor measuring device which detects vibration occurring at the beginning of slippage.
Specifically, for example, Non-Patent Document 1 discloses a pressure-sensing tactile sensor having silicon resin with a curved surface. Inside the silicon resin, a number of strain sensors are provided. This tactile sensor estimates a friction factor or the like, based on how the stress inside the sensor varies before the gripped object starts to slide. Another exemplary pressure-sensing tactile sensor is disclosed in Non-Patent Document 2. This document discloses a tactile sensor with a fingertip having a curved surface supported by plural springs. This tactile sensor estimates a friction factor or the like, based on how internal stress varies inside the sensor before the gripped object starts to slide.
Further, Patent Document 1 discloses a pressure-sensing tactile sensor which uses a hole and ultrasonic wave. The document describes a friction factor measuring method and a maximum transverse-displacement force measuring method as follows. Namely, in these methods, at the moment when the tactile sensor is pushed against a target, the tactile sensor observes the stress and a strain component in the direction along a surface of the target. In this manner, the friction factor and the maximum transverse-displacement force are stably detected.
Although Patent Document 2 discloses no pressure-sensing tactile sensor, it discloses an image processing algorithm as a technology relative to a tactile sensor. The document describes that a contact surface of a target and a transparent-gel-made sensing section of the sensor is observed by using a camera, and variation in the shape of the sensing section is estimated using the image processing algorithm, based on the obtained image information.
Further, Patent Document 3 discloses a portable friction gauge for estimating the friction factor as follows. A device for estimating the friction factor is placed on a floor face, and a force is increased little by little in the tangential direction to the device. Then based on a force at the time when there occurs slippage, the gauge of Patent Document 3 estimates the friction factor.
[Patent Document 1]
Japanese Unexamined Patent Publication No. 2001-021482 (Tokukai 2001-021482; Published on Jan. 26, 2001)
[Patent Document 2]
Specification of U.S. Pat. No. 5,967,990
[Patent Document 3]
Japanese Unexamined Patent Publication No. 1995-151672 (Tokukaihei 07-151672; Published on Jun. 16, 1995)
[Non-Patent Document 1]
Kawai Takashi and two others, “Development of Strain Distribution Sensor Having Curved Surface for Grip Force Control”, The Japan Society of Mechanical Engineers, Collected Papers C Edition, Vol. 64, No. 627, pp. 4264-4270, 1998
[Non-Patent Document 2]
Fujimoto Hideo, Sano Akihito, Nishitsune Kai, Uehara Yusaku, “Tactile Sensor Mounted Soft Finger for Remote Gripping with a Multi-fingered Hand”, Lecture on Robotics and Mechatronics, 2002
The above-mentioned pressure-sensing tactile sensors have the following problems. First, regarding the tactile sensor disclosed in Non-Patent Document 1, the number of strain sensors which can be provided in silicon resin is limited. Provision of tactile sensors needed for achieving sufficient detection sensibility increases the size of the device. On the other hand, downsizing of the device will cause insufficient detection accuracy.
Similarly, regarding the tactile sensor of Non-Patent Document 2, the number of springs which can be provided inside the sensor is limited. Therefore, sufficient detection accuracy is not obtained.
Further, in regard to the tactile sensor of Patent Document 1, the number of holes which can be provided in the sensor is limited. This causes a problem in the detection accuracy. Additionally, prior to the use of such a sensor, phase variation of the friction factor and ultrasonic wave needs to be measured a number of times to prepare a table that grasps the interrelation therebetween. This makes the use of the sensor extremely troublesome.
Further, although the technology of Patent Document 2 is relative to a tactile sensor, it is not relative to detection of a slippage margin or a friction factor. Therefore, measurement of the slippage margin, friction factor, or the like is not possible with the technology alone.
Further, in the measurement of the friction factor with the use of the portable friction gauge of Patent Document 3, it is necessary to cause an entire slippage state in which an object to be measured actually starts to slip. For this reason, measurement of the friction factor or the like with the portable friction gauge is not easy. Furthermore, since the object needs to be actually slid when using the portable friction gauge, the gauge is only usable for a plane having a certain degree of broadness. This is a problem in terms of general use.
As described above, the pressure-sensing tactile sensor requires a number of stress sensors to accurately detect the friction factor or slippage between a measurement target and the sensing section of the sensor. However, since the device size needs to be reduced, the number of stress sensors which can be provided is limited due to the space of the device. Therefore, sufficient detection accuracy is not obtained. Furthermore, there is development of a technology of detecting, with a use of a camera, variation in the contact surface of an elastic member and a rigid object. However, there is no technology to apply the technology to measurement of slippage and the friction factor between objects.
Under the circumstances, there has been a strong demand for a tactile sensor and a method of using the same, to solve the above problems and allow for easy but accurate measurement of a slippage margin or a friction factor between objects.